Overview
The battery pack for the LifeKart is going to be the largest and least configurable portion. This is due to the large Lifepo4 cells that don't provide as many options for different layouts or connection sequences as smaller Lithium Ion cells.
Because of this we are going to design the LifeKart chassis and component layout around the battery pack. This ensure everything is balanced and makes sense.
Voltage and Series Connections
This pack will be configured to run 96 volts. This means a 32Sx1P cell layout. But how did we determine this? Using simple math we can calculate how many series cells are needed to achieve 96v.
It's important to remember when calculating pack voltage to use the nominal voltage for the specific chemistry of your cells.
The LifeKart battery will be using Lifepo4 cells which carry a nominal voltage of around 3.2 volts. With the equation below we determine a need of 30 cells at nominal voltage to get 96v.
96v / 3.2v = 30 cells.
This is all well and good but what happens when the pack is less than half charged? Let's say 3v or less. Well we can use the same equation above just flipped.
30 cells X 3v = 90v
As seen above we are below our 96v goal. This isn't "bad" persay but our pack wont be able to put out the same "punch" kilowatt wise when heavy on the throttle at this lower voltage. However, we can easily compensate for this by throwing in a couple more series cells to the pack. The math looks something like below
96v / 3v = 32 cells.
By adding these two extra cells there is more power in the pack at our desired 96v. 20% more to be exact.
30 cells = 96v at 70% or greater charge
32 cells = 96v at 50% or greater charge
The cell voltage numbers above are an average of the entire market. Some Lifepo4 cells are happy at 3.6v and some will drain to 2.4 without any issues.
Two Is Better Than One
The LikeKart battery cells are quite large compared to typically used Lithium Ion cylindrical cells. The photo below shows visual scale.
This means bumping 32 of them next to each other makes a pretty big battery. In fact there really isn't anywhere to put that large of a battery on a go-kart without a very long frame and sticking it behind the seat which is not desirable.
So it will be cut in half! That's right, the final battery pack will actually be two separate modules of 16 cells and some custom plugs to link it all together. This will allow mounting of each battery module on either side of the seat. Future posts will cover all of the wiring and assembly required.
Below is a crude visual of the above description.
Conclusion
I hope this gives a better idea of the LifeKart battery pack layout, design and strategy behind these decisions. Stay tuned for the next post where we'll start sizing up the cells and planning out the pack assembly.